1. Field of the Invention
The present invention relates to a reflective liquid crystal display device, and more particularly to a reflective cholesteric liquid crystal display device.
2. Description of the Related Art
The liquid crystal display device primarily includes a pair of substrates, between which a liquid crystal layer is held. When a predetermined drive voltage is applied to the liquid crystal layer to control orientations of liquid crystal molecules in the liquid crystal layer, external light incident on the liquid crystal display device is modulated to perform intended display of images or the like. The liquid crystal display device using the cholesteric liquid crystal has been known. Examples of the cholesteric liquid crystal are, e.g., liquid crystal, which exhibits the cholesteric phase by itself, and chiral nematic liquid crystal obtained by adding a chiral agent to nematic liquid crystals. The cholesteric liquid crystal has such a feature that the liquid crystal molecules forms helical structures, and can exhibit three states, i.e. a planar state, focal conic state and a homeotropic state when it is held between a pair of substrates, and is subjected to an external field. In the liquid crystal display device using the cholesteric liquid crystals, these three states exhibit different light transparencies and reflectances. Therefore, the three states and the manner of applying the external electric field can be appropriately selected to perform the display. For example, the display may be performed in the cholesteric-nematic phase transfer mode using the homeotropic state and the focal conic state, and maybe performed in a bistable mode using the planar state and the focal conic state.
Among them, the display in the bistable mode has such a feature that the planar state and the focal conic state are stable even in the state where no external electric field is applied, and thus has the bistability, which maintains the display state even when no external electric field is applied. The liquid crystal display device using the cholesteric liquid crystals exhibits the selective reflection property in the visible wavelength range when it is in the planar state, and achieves a bright reflection state. In other words, it can perform bright display without using a polarizing plate or a color filter.
The cholesteric liquid crystal having the characteristics of bistability and high reflectivity is very suitable for application in the development of the reflective black and white liquid crystal display device. Nevertheless, the reflective spectrum of the cholesteric liquid crystals is changed as different orientations of the liquid crystal molecules in the liquid crystal layer. It is an important issue to improve the wide viewing angle behavior of the liquid crystal display devices. The wavelengths of the reflective light are relevant to the pitches of the cholesteric liquid crystals. It is desired for making the helical axes of the cholesteric liquid crystal molecules in the liquid crystal layer orientate in multi-directions such that the cholesteric liquid crystals in the liquid crystal layer may have various effective pitches and the reflective spectrum can be broadened. Referring to FIG. 1, U.S. Pat. No. 6,809,788 discloses that the surfaces of the substrates relative to the liquid crystal layer are processed to make the cholesteric liquid crystal molecules proximate to the processed surfaces of the substrates orientate in multi-directions but have chaotic arrangement. U.S. Pat. No. 6,809,788 only can make the helical axes of the cholesteric liquid crystal molecules proximate to the surfaces of the substrates orientate in declining directions, but can not affect the cholesteric liquid crystal molecules in the interior of the liquid crystal layer. There is limited effect to broaden the viewing angle.
Accordingly, it is an intention to provide an improved cholesteric liquid crystal display structure, which can overcome the above drawbacks.